This invention concerns an SZ-spiral-grooved spacer for optical fiber cable, an optical fiber cable that uses this spacer, and a method for producing this spacer, and in particular, concerns an SZ-spiral-grooved spacer, which is made thin in diameter as a result of the groove inclination at the inversion parts being restricted even though the minimum rib thickness is 1.0 mm or less.
The making of optical fiber cables thin in diameter, lightweight, and high in optical wiring density is being pursued to reduce cable prices and laying costs, and there have been stringent demands for making polyethylene (PE) spacers, which accommodate optical fibers, thin in diameter as well.
Meanwhile, with recent aerial optical fiber cables, intermediate post-branching performance of the optical fibers is being required in addition to high optical wiring densities, and this has lead to the frequent use of SZ-type optical fiber cables, which use a PE spacer (SZ spacer), having grooves that accommodate the optical fibers and are alternately inverted in spiraling direction in SZ-like manner, and with which a plurality of tape-form optical fibers are accommodated within each groove of the spacer.
In the case where a rigid optical tape is to be accommodated used in an SZ spacer, the dimensions of an accommodating groove must enable the securing of adequate space for allowing the twisting of the tape. Also, though the polyethylene resin that comprises the rib undergoes three-dimensional molding shrinkage (sum of the shrinkage due to recrystallization during solidification and volume shrinkage due to lowering of the resin temperature) in the process of extrusion molding, unlike in the case of a unidirectionally stranded spacer, with which there is no allowance for shrinkage of the ribs in the length direction, in the case of an SZ spacer, lengthwise shrinkage of the ribs is possible in the form of short-cutting the inversion curve at just the inversion part, and as a result, the ribs can collapse towards the inner side of the inversion curve.
This phenomenon becomes more prominent when the ribs are made thin in root thickness and, along with the abovementioned securing of groove space, this has been a factor that has hampered the making of SZ slots thin in diameter.
It is considered that besides the molding shrinkage of the resin, the collapsing of the ribs may be caused by the mutual pulling of the coating resins, due to differences in the drawdown of the resin, etc., in the process of performing extrusion coating from a nozzle.
In the case of an optical fiber cable that uses a thin-diameter SZ spacer, with which the minimum rib thickness at the root, etc. of the rib is thin, the inversion pitch must be made short in order to allow for extra lengths of optical fiber, and since the inclination angle of the groove at the inversion part thus becomes large, the transmission loss is increased inevitably.
An object of this invention is to provide an SZ-spiral-grooved spacer for optical fiber cable, with which the groove inclination at the inversion parts of the SZ spacer is restricted and is low in the increase of transmission loss, and an optical fiber cable that uses this spacer to realize the making of optical fiber cables thin in diameter.
In order to achieve the above object, this invention provides in a polyethylene spacer for optical fiber cable, with which a thermoplastic resin, with compatibility with polyethylene, is applied as an intermediate coating layer onto the periphery of a central tensile member and with which a main coating, having continuous spiral grooves that are for accommodating optical fibers and are inverted periodically in direction along the length direction, is formed from polyethylene on the outer periphery of the abovementioned intermediate coating layer, a spacer such that the minimum rib thickness of the ribs that define the abovementioned spiral grooves is 1.0 mm or less and the groove inclination angle in the spacer cross section of the inversion parts is 18xc2x0 or less.
This invention also provides in a polyethylene spacer for optical fiber cable, with which a main coating, having continuous spiral grooves that are for accommodating optical fibers and are inverted periodically in direction along the length direction, is formed from polyethylene on the outer periphery of a central tensile member, a spacer such that the minimum rib thickness of the ribs that define the abovementioned spiral grooves is 1.0 mm or less and the groove inclination angle in the spacer cross section of the inversion parts is 18 or less.
With the spacer of the above arrangement, the resin density of the portions substantially at the roots of the ribs that define the abovementioned spiral grooves may be made the smallest in comparison to the tip parts of the ribs and the central parts of the ribs.
With the spacer of the above arrangement, the average roughness of the groove bottoms of the abovementioned spiral grooves may be made 1.2 xcexcm or less.
With the spacer of the above arrangement, the spiral progression angle (xcex2), as determined by
tanxcex2=(dxc3x97xcfx80xc3x97xcex8/360)/p
where d is the outer diameter, xcex8 is the spiral groove inversion angle, and p is the spiral groove inversion pitch, may be set in the range, 5 to 15xc2x0.
Also with the present invention, the spacer of the above arrangement may be used to accommodate a plurality of tape-form optical fibers in at least one or more spiral grooves to form an optical fiber cable.
Also with the present invention, the spacer of the above arrangement may be used to accommodate a single-core optical fiber in at least one or more spiral grooves to form an optical fiber cable.
Furthermore, the present invention provides in a method for producing a polyethylene spacer for optical fiber cable, with which a thermoplastic resin, with compatibility with polyethylene, is applied as an intermediate coating layer onto the periphery of a central tensile member and with which a polyethylene main spacer coating, having continuous spiral grooves that are for accommodating optical fibers and are inverted periodically in direction along the length direction, is formed on the outer periphery of the abovementioned intermediate coating layer, a production method wherein after the abovementioned main spacer coating spacer is applied, a cooling medium is blown, obliquely at a predetermined angle with respect to the running direction of the abovementioned spacer, onto the outer periphery of the abovementioned spacer.
This invention also provides in a method for producing a polyethylene spacer for optical fiber cable, with which a polyethylene main spacer coating, having continuous spiral grooves that are for accommodating optical fibers and are inverted periodically in direction along the length direction, is formed on the periphery of a central tensile member, a production method wherein after the abovementioned main spacer coating spacer is applied, a cooling medium is blown, obliquely at a predetermined angle with respect to the running direction of the abovementioned spacer, onto the outer periphery of the abovementioned spacer.
This invention also provides in a method for producing a polyethylene spacer for optical fiber cable, with which a polyethylene main spacer coating, having continuous spiral grooves that are for accommodating optical fibers and are inverted periodically in direction along the length direction, is formed on the outer periphery of a central tensile member, a production method wherein a reinforced fiber bundle, which comprises the abovementioned tensile member, is drawn upon being impregnated with an uncured thermosetting resin, then upon inserting this reinforced fiber bundle into a melt extrusion molding die, a polyethylene resin is extruded and coated onto the outer periphery, then after cooling the coated resin on the surface, the thermosetting resin in the interior is cured, and then after applying the abovementioned main spacer coating onto the outer periphery of the abovementioned coating resin, a cooling medium is blown, obliquely at a predetermined angle with respect to the running direction of the abovementioned spacer, onto the outer periphery of the abovementioned spacer.
With the method for producing a spacer for optical fiber cable of the above-described arrangement, the cooling medium may be warm water of 40xc2x0 C. or more to which a surfactant has been added.
Also with the method for producing a spacer for optical fiber cable of the above-described arrangement, the cooling medium may be dry air or may be moist air, including mist.
Furthermore with the above-described production method, the abovementioned predetermined angle may be set to an angle of within 30xc2x0 to 150xc2x0.
Also with the above-described method for producing a spacer for optical fiber cable, optical fibers may be accommodated in the abovementioned spiral grooves after cooling and solidifying the abovementioned main spacer coating by the blowing on of an abovementioned cooling medium and a sheath coating may be provided by press winding a non-woven fabric around the outer periphery to produce an optical fiber cable.
With this optical fiber cable production method, if a spacer with which the inclination of the spiral grooves is restricted is to be obtained, it will be effective to employ the method of reheating the spacer, obtained after application of the main spacer coating, to a temperature of 60xc2x0 or more and yet less than or equal to the melting point while applying tension and inserting a sizing device, etc. that inserts a pin, etc., into the spiral grooves.
The central tensile member that can be used in this invention is not restricted in particular, and may be a single steel wire, stranded steel wire, single FRP wire, stranded FRP wire, polymer tensile member, etc. that is selected according to the tensile strength, flexibility, lightweightness, economy, etc. that are in accordance with the tensile strength required of an optical fiber cable.
The intermediate coating layer of thermoplastic resin at the outer periphery of the tensile wire must be bonded to or strongly adhered to the abovementioned tensile wire in the case where the tensile wire is a single wire but in the case where the tensile wire is a stranded wire and anchor adhesion by the stranded structure can be anticipated, bonding may not be necessary.
As the thermoplastic resin to be used in the intermediate coating layer, a resin, which has mutual compatibility with the polyethylene resin (to be referred to as the xe2x80x9cmain spacer coating resinxe2x80x9d) that is coated onto the outer periphery of the intermediate coating layer and forms the grooves, is selected.
Here, having compatibility means that the thermoplastic resin of the intermediate coating layer and the main spacer coating resin are mutually high in compatibility and are in a relationship where melt adhesion is possible or in a relationship where bonding to some degree is possible by the use of an adhesive, solvent, etc.
In the case where a high-density, medium-density, or low-density polyethylene is selected as the main spacer coating resin, a resin of the same type or a modified resin of the above, etc. is used as the resin for the intermediate coating resin.
With the polyethylene optical fiber spacer by this invention, a known heat-resistant stabilizer, age resister, anti-weathering stabilizer, hydrochloric acid absorber, lubricant, organic or inorganic pigment, carbon black, gum resister, fire retardant, antistatic agent, filler, etc. may be added to the polyethylene resin.
Furthermore, as the need arises, an eutectic copolymer resin of a cyclic olefin and ethylene, an alloy resin, a modified polyethylene resin, or a crosslinked polyethylene resin may be mixed.
The continuous spiral grooves for accommodating the optical fiber, which are inverted periodically in direction along the length direction, are formed by melt extruding and coating a polyethylene resin, and the inversion angle (xcex8) and period of inversion (inversion pitch p) of the spiral grooves are designed according to the specifications of the optical fiber cable.
In general, an inversion angle (xcex8) of 275xc2x0xc2x15 is deemed to be preferable as indicated in Japanese patent publication No. 13687 of 1995, and with the present invention, the inversion angle is also selected within the range, 200 to 375xc2x0, which is centered about the abovementioned inversion angle.
With the spacer for thin-diameter optical fiber cable by this invention, the minimum rib thickness of the ribs that define the spiral grooves is 1 mm or less. When the minimum rib thickness exceeds 1 mm, the proportion of the cross section of the spacer taken up by the groove part becomes small, making it difficult to achieve a thin diameter and a high density. A minimum rib thickness of 0.9 mm or less is therefore even more preferable.
As shown in FIG. 4, the groove inclination angle refers to the angle indicated by the narrow angle a formed by the line L1, which joins the spacer center O and the groove bottom center A in the cross section of the inversion part of the SZ spacer, and the line L2, which joins the groove bottom center A and the center B of the outer width of the groove, and is measured from an enlarged photograph of the cross section of the spacer.
When an optical fiber is housed with the groove inclination angle xcex1 at the inversion part exceeding 18xc2x0, the transmission loss tends to increase. The allowable range was thus limited to 18xc2x0 or less.
Also in terms of restricting the groove inclination angle a at the inversion part to 18xc2x0 or less, the spacer of this invention is preferably a spacer for optical fiber cable with which the resin density at substantially the root parts of the ribs that define the spiral grooves is the lowest in comparison to those of the tip parts of the ribs and the central parts of the ribs.
The resin density at the root parts can be made smaller than those at the tip parts of the ribs and the central parts of the ribs by performing the cooling and solidification of the root parts at an early stage, and as a result of this, the root parts become lower in crystallinity and relatively lower in resin density than the gradually cooled central parts and tip parts of the ribs.
Thus with the production method of this invention, a cooling medium is blown, obliquely at a predetermined angle with respect to the running direction of the abovementioned spacer, onto the spacer in the process of applying the main spacer coating, which has continuous spiral grooves for accommodating the optical fiber that are inverted periodically in direction along the length direction, onto the outer periphery of the intermediate coating layer that is coated onto the tensile wire.
With the main spacer coating, it is considered that the spacer that is melt extruded in the prescribed form with grooves and ribs is in the condition where it is surrounded by a high-temperature sheath with a temperature gradient ranging from the melt resin temperature to the ambient temperature, and in view that this temperature sheath must be peeled off to promote cooling and cause solidification by blowing on a cooling medium, the cooling medium is blown onto the outer periphery of the spacer to cause this peeling off of the temperature sheath to occur at an early stage at the groove parts.
Thus after melt extrusion from the die, the cooling medium is blown onto the bottom part of the groove in the case where the cooling medium is air, mist, etc. On the other hand, if the cooling medium is a liquid, it must be made to contact the groove.
When the cooling medium is blown onto the groove bottom, the root parts of the ribs that are positioned at the sides of a groove bottom are cooled earlier and with more priority than the intermediate parts of the rib. When the root parts or the ribs are cooled in this manner, the shapes of the ribs will be stabilized at an early stage and inclination thereof can be prevented effectively.
In the case where the cooling medium is a liquid, warm water of 40xc2x0 C. or more, to which a surfactant has been added, is preferable from the point of economy since washing treatment, etc. are unnecessary after the process.
A surfactant is added since if only warm water without surfactant is used, air bubbles become attached to the surface of the main spacer and traces of these bubbles will remain as so-called blobs after cooling and solidification.
The temperature of the warm water is set to 40xc2x0 C. or more since at a temperature of less than 40xc2x0 C., rapid cooling will occur and unfavorable vacuum voids, etc. will form in the main spacer.
The cooling medium may be dry air or moist air, including mist. In the case where mist is to be used, it is preferable to adjust the mist concentration to a concentration in a range at which the mist will vaporize and not aggregate on the groove walls, etc. and at which the particle diameter of the mist will have a cooling effect and will not lead to visible attachment marks.
In blowing or contacting the cooling medium onto the groove bottoms of the spiral grooves, the cooling medium is blown at a predetermined rate and obliquely at an angle of 30xc2x0 to 150xc2x0 with respect to the longitudinal axis of the running spacer.
At an angle of blowing that is less than 30xc2x0 or greater than 150xc2x0, the flow of the cooling medium becomes an adjoint flow or counterflow that is parallel to the spacer, preventing the cooling medium from flowing effectively to the groove parts, significantly lowering the action of peeling off the high-temperature sheath formed around the main spacer, and making it difficult to restrict the groove inclination angle xcex1 at the inversion parts to 18xc2x0 or less.
With this invention""s method for producing a spacer for optical fiber cable, the drawdown is preferably set to 70% or less. Here, with the main spacer coating, the drawdown is defined as ([Sb/Snb]xc3x97100) where Sb is the cross-sectional area of the main spacer that is formed and Snb is the actual cross-sectional area of resin discharge from the nozzle (the cross-sectional area obtained by subtracting the cross-sectional area of the coated tensile wire from the area of the nozzle opening).
When the drawdown is set to 70% or more or even more preferably to 90% or more and the die land area is set to a prescribed length, melt fracture is prevented, the mutual pulling of resins caused by the drawdown of the resin during discharge is relaxed, etc.
Furthermore, with regard to the mechanical characteristics of the main spacer coating resin, it is preferable for the flexural modulus to be 490 Mpa or more in order to prevent deformation of the ribs due to the making of the rib thickness thin in accompaniment with the making of the cable thin in diameter.
It is important for the spacer for optical fiber cable to have a prescribed flexibility in order to facilitate handling in the process of actually laying an optical fiber that uses the spacer for optical fiber cable.
For example, in the case where the spacer is to be used in an aerial optical cable, if the spacer for optical fiber cable is poor in flexibility, not only will this have an effect on production of the optical fiber but the laying of the optical cable is made difficult, etc. as indicated in laid-open Japanese patent publication No. 113932 of 1995.
Thus with regard to the mechanical characteristics of the intermediate coating resin, the flexural modulus is set to 98 to 490 Mpa to avoid such effects.
With the spacer of this invention, the groove bottoms of the spiral grooves are preferably made 1.2 xcexcm or less in average roughness. This is because the optical fiber or tape core wire that is accommodated within the spiral grooves of the spacer come in direct contact with the groove bottoms and if the surface roughness of the groove is large, microbending occurs in the optical fiber, which leads to increased transmission loss, especially in the long wavelength range (xcex=1.55 xcexcm). This problem can be resolved by making the surface roughness 1.2 xcexcm or less.